The invention relates to the field of absorbers of electromagnetic energy, and in particular to a passive foam-fiber radar absorber.
To test radar systems and other RF transmitters or receivers, passive absorbers have long been used to cover reflective walls inside a test chamber (e.g., an anechoic chamber). Generally, the principle objective of these absorbers is to coat reflective surfaces so any incident RF energy that strikes the absorber is largely absorbed and attenuated, rather than being reflected. The absorbers create an environment having no reflective boundaries so radar systems and antennas can be tested as if you are testing in an open field. Absorbers are also used on naval vessels and military aircraft to reduce radar cross section (RCS).
In order to capture the RF energy, the best performing absorbers are generally pyramid shaped. This shape provides a gradual impedance transition which facilitates absorbing RF energy. Resistive material within the absorber converts the RF energy to heat which is dissipated. Absorbers are available for a wide range of frequencies (e.g., 10 MHz-100 GHz).
To form a pyramid shaped absorber one basically starts with a low density polyurethane foam, such as furniture grade foam. The foam is then immersed in an aqueous dispersion that includes carbon black and a binder material. Specifically, the foam is placed between a pair of parallel plates that are squeezed tight, and then submerged in a tank containing the aqueous dispersion. The plates are opened and closed several times so the carbon dispersion can be squeezed into the foam, analogous to a sponge. The foam is then raised above the surface, squeezed to remove excess solution and dried in a oven. Once dry, the foam is trimmed to the final shape.
There are a number of problems with this process. First, the carbon black film deposited onto the surface of the foam cells is a difficult material to control with respect to electrical resistance. For example, the resistance of carbon black varies lot-to-lot. In addition, because of the difference in pressure applied by the plates to the foam, there may be a higher concentration of carbon in the center of the foam versus the outside, or vice versa. Furthermore, there is generally a gravity gradient caused by migration of the dispersion of carbon black as the foam dries, and as a result more carbon is located at the base of the piece. Therefore, it is very difficult to realize an absorber having uniform resistance.
Another problem with prior art absorbers is cost. Top quality furniture grade foam is required, and this foam is expensive. In addition, the energy cost to dry the wet piece of foam is relatively high.
Yet another problem with prior art absorbers is that they are full of carbon black that is capable of burning and smoldering. As a result, a significant amount of fire retardant material has to be added. Typically 75% of the absorber weight and raw material cost relates to the fire retardant material.
Furthermore, the absorber shape has been limited to geometries which are attainable using an abrasive saw or a hot wire cutter. This significantly limits how the material can be shaped.
Therefore, there is a need for a reduced cost, uniform electromagnetic absorber.